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This is a little bit off the original topic, but I have a question.

Given two unmarked pieces of coax, how do you determine if they're "50
ohm" or "75 ohm" or whatever? Shorting them doesn't work.

Jack.
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Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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Use an antenna analyzer!

At 1/8 wavelength, from either a short or an open, the input impedance
will be purely resistive. The resistive value is the Zo of the line.

DD, W1MCE

Jack Twilley wrote:

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This is a little bit off the original topic, but I have a question.

Given two unmarked pieces of coax, how do you determine if they're "50
ohm" or "75 ohm" or whatever? Shorting them doesn't work.

Jack.
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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"Dave" =3D=3D Dave Shrader writes:

Dave Use an antenna analyzer!

Umm, assume that instead of an antenna analyzer, I've got a VOM.

Dave At 1/8 wavelength, from either a short or an open, the input
Dave impedance will be purely resistive. The resistive value is the
Dave Zo of the line.

Without being able to send a signal, I won't be able to test it?

Jack.
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Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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NO! You can't test it with a VOM. You need a signal with wavelength
[frequency] characteristics.

Someone in your area should have an MFJ 259B. Ask around.

Deacon Dave, W1MCE

Jack Twilley wrote:

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"Dave" == Dave Shrader writes:


Dave Use an antenna analyzer!

Umm, assume that instead of an antenna analyzer, I've got a VOM.

Dave At 1/8 wavelength, from either a short or an open, the input
Dave impedance will be purely resistive. The resistive value is the
Dave Zo of the line.

Without being able to send a signal, I won't be able to test it?

Jack.
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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Dave Shrader wrote:
At 1/8 wavelength, from either a short or an open, the input impedance
will be purely resistive. The resistive value is the Zo of the line.

Don't you mean purely *reactive*, Dave, like 0+jZ0 or 0-jZ0?
--
73, Cecil http://www.qsl.net/w5dxp



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Without any RF test equipment, another way to tell is by physical
measurement of the dielectric and inner conductor diameters. But you'll
need some calipers to do that. If you have piece of marked cable of the
same outside diameter and either impedance for comparison, you can
easily tell the difference -- for the same outside diameter, 75 ohm
cable has a noticeably smaller inner conductor than 50 ohm cable.

Roy Lewallen, W7EL

Jack Twilley wrote:
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This is a little bit off the original topic, but I have a question.

Given two unmarked pieces of coax, how do you determine if they're "50
ohm" or "75 ohm" or whatever? Shorting them doesn't work.

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"Roy" =3D=3D Roy Lewallen writes:

Roy Without any RF test equipment, another way to tell is by physical
Roy measurement of the dielectric and inner conductor diameters. But
Roy you'll need some calipers to do that. If you have piece of marked
Roy cable of the same outside diameter and either impedance for
Roy comparison, you can easily tell the difference -- for the same
Roy outside diameter, 75 ohm cable has a noticeably smaller inner
Roy conductor than 50 ohm cable.

This is very helpful. Thanks!

Jack.
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Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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